In one known apparatus for measuring the amount of yarn withdrawn from the warp beam of a warp knitting machine, a pressure roll engages the warp beam and drives directly a tachogenerator which emits pulse signals representing the amount, that is, the length of yarns withdrawn from the warp beam. The pulse signals may then be used in a known, conventional manner for controlling the drive mechanism of the warp beam.
In this one known apparatus, the pressure roll is mounted on a support arm which is biased toward the warp beam so that the pressure roll engages the warp beam. When the warp beam is changed, the support arm pivots away from the warp beam and the pressure roll disengages from the warp beam and stops rotating. When the support arm is pivoted in this manner during normal operation of a warp knitting machine, the drive mechanism of the warp beam is abruptly interrupted as soon as pivotal support arm motion begins because the stoppage of the pressure roll rotation simulates an absence of yarn feed. As a consequence of the abrupt termination of pressure roll rotation, the warp beam drive accelerates, leading to a sagging of the yarns and an immediate breakdown in the warp knitting machine operation. Additionally, sometimes during knitting, the support arm may be moved slightly from operator error or machine vibration, thus creating the same problem.